$ A = \left[\begin{array}{rr}0 & 2 \\ 1 & -2\end{array}\right]$ $ F = \left[\begin{array}{rr}2 & 0 \\ 4 & 5\end{array}\right]$ What is $ A F$ ?
Answer: Because $ A$ has dimensions $(2\times2)$ and $ F$ has dimensions $(2\times2)$ , the answer matrix will have dimensions $(2\times2)$ $ A F = \left[\begin{array}{rr}{0} & {2} \\ {1} & {-2}\end{array}\right] \left[\begin{array}{rr}{2} & \color{#DF0030}{0} \\ {4} & \color{#DF0030}{5}\end{array}\right] = \left[\begin{array}{rr}? & ? \\ ? & ?\end{array}\right] $ To find the element at any row $i$ , column $j$ of the answer matrix, multiply the elements in row $i$ of the first matrix, $ A$ , with the corresponding elements in column $j$ of the second matrix, $ F$ , and add the products together. So, to find the element at row 1, column 1 of the answer matrix, multiply the first element in ${\text{row }1}$ of $ A$ with the first element in ${\text{column }1}$ of $ F$ , then multiply the second element in ${\text{row }1}$ of $ A$ with the second element in ${\text{column }1}$ of $ F$ , and so on. Add the products together. $ \left[\begin{array}{rr}{0}\cdot{2}+{2}\cdot{4} & ? \\ ? & ?\end{array}\right] $ Likewise, to find the element at row 2, column 1 of the answer matrix, multiply the elements in ${\text{row }2}$ of $ A$ with the corresponding elements in ${\text{column }1}$ of $ F$ and add the products together. $ \left[\begin{array}{rr}{0}\cdot{2}+{2}\cdot{4} & ? \\ {1}\cdot{2}+{-2}\cdot{4} & ?\end{array}\right] $ Likewise, to find the element at row 1, column 2 of the answer matrix, multiply the elements in ${\text{row }1}$ of $ A$ with the corresponding elements in $\color{#DF0030}{\text{column }2}$ of $ F$ and add the products together. $ \left[\begin{array}{rr}{0}\cdot{2}+{2}\cdot{4} & {0}\cdot\color{#DF0030}{0}+{2}\cdot\color{#DF0030}{5} \\ {1}\cdot{2}+{-2}\cdot{4} & ?\end{array}\right] $ Fill out the rest: $ \left[\begin{array}{rr}{0}\cdot{2}+{2}\cdot{4} & {0}\cdot\color{#DF0030}{0}+{2}\cdot\color{#DF0030}{5} \\ {1}\cdot{2}+{-2}\cdot{4} & {1}\cdot\color{#DF0030}{0}+{-2}\cdot\color{#DF0030}{5}\end{array}\right] $ After simplifying, we end up with: $ \left[\begin{array}{rr}8 & 10 \\ -6 & -10\end{array}\right] $